path: root/epan/dissectors/packet-null.c

/* packet-null.c
 * Routines for null packet disassembly
 *
 * Wireshark - Network traffic analyzer
 * By Gerald Combs <gerald@wireshark.org>
 *
 * This file created by Mike Hall <mlh@io.com>
 * Copyright 1998
 *
 * SPDX-License-Identifier: GPL-2.0-or-later
 */

#include "config.h"

#include <wsutil/pint.h>

#include <epan/packet.h>
#include <epan/capture_dissectors.h>
#include "packet-ip.h"
#include "packet-ppp.h"
#include <epan/etypes.h>
#include <epan/aftypes.h>

void proto_register_null(void);
void proto_reg_handoff_null(void);

static dissector_table_t null_dissector_table;
static dissector_table_t ethertype_dissector_table;

/* protocols and header fields */
static int proto_null = -1;
static int hf_null_etype = -1;
static int hf_null_family = -1;

static gint ett_null = -1;

/* Null/loopback structs and definitions */

/* Family values. */
static const value_string family_vals[] = {
  {BSD_AF_INET,          "IP"             },
  {BSD_AF_ISO,           "OSI"            },
  {BSD_AF_APPLETALK,     "Appletalk"      },
  {BSD_AF_IPX,           "Netware IPX/SPX"},
  {BSD_AF_INET6_BSD,     "IPv6"           },
  {BSD_AF_INET6_FREEBSD, "IPv6"           },
  {BSD_AF_INET6_DARWIN,  "IPv6"           },
  {0,                    NULL             }
};

static dissector_handle_t null_handle, loop_handle;
static capture_dissector_handle_t null_cap_handle;

static dissector_handle_t ppp_hdlc_handle;
static capture_dissector_handle_t ppp_hdlc_cap_handle;

static gboolean
capture_null( const guchar *pd, int offset _U_, int len, capture_packet_info_t *cpinfo, const union wtap_pseudo_header *pseudo_header _U_ )
{
  guint32 null_header;

  /*
   * BSD drivers that use DLT_NULL - including the FreeBSD 3.2 ISDN-for-BSD
   * drivers, as well as the 4.4-Lite and FreeBSD loopback drivers -
   * stuff the AF_ value for the protocol, in *host* byte order, in the
   * first four bytes.
   *
   * However, the IRIX and UNICOS/mp snoop socket mechanism supplies,
   * on loopback devices, a 4-byte header that has a 2 byte (big-endian)
   * AF_ value and 2 bytes of 0, so it's
   *
   *    0000AAAA
   *
   * when read on a little-endian machine and
   *
   *    AAAA0000
   *
   * when read on a big-endian machine.  The current CVS version of libpcap
   * compensates for this by converting it to standard 4-byte format before
   * processing the packet, but snoop captures from IRIX or UNICOS/mp
   * have the 2-byte+2-byte header, as might tcpdump or libpcap captures
   * with older versions of libpcap.
   *
   * AF_ values are small integers, and probably fit in 8 bits (current
   * values on the BSDs do), and have their upper 24 bits zero.
   * This means that, in practice, if you look at the header as a 32-bit
   * integer in host byte order:
   *
   *    on a little-endian machine:
   *
   *            a little-endian DLT_NULL header looks like
   *
   *                    000000AA
   *
   *            a big-endian DLT_NULL header, or a DLT_LOOP header, looks
   *            like
   *
   *                    AA000000
   *
   *            an IRIX or UNICOS/mp DLT_NULL header looks like
   *
   *                    0000AA00
   *
   *    on a big-endian machine:
   *
   *            a big-endian DLT_NULL header, or a DLT_LOOP header, looks
   *            like
   *
   *                    000000AA
   *
   *            a little-endian DLT_NULL header looks like
   *
   *                    AA000000
   *
   *            an IRIX or UNICOS/mp DLT_NULL header looks like
   *
   *                    00AA0000
   *
   * However, according to Gerald Combs, a FreeBSD ISDN PPP dump that
   * Andreas Klemm sent to wireshark-dev has a packet type of DLT_NULL,
   * and the family bits look like PPP's protocol field.  (Was this an
   * older, or different, ISDN driver?)  Looking at what appears to be
   * that capture file, it appears that it's using PPP in HDLC framing,
   * RFC 1549, wherein the first two octets of the frame are 0xFF
   * (address) and 0x03 (control), so the header bytes are, in order:
   *
   *    0xFF
   *    0x03
   *    high-order byte of a PPP protocol field
   *    low-order byte of a PPP protocol field
   *
   * If we treat that as a 32-bit host-byte-order value, it looks like
   *
   *    PPPP03FF
   *
   * where PPPP is a byte-swapped PPP protocol type if we read it on
   * a little-endian machine and
   *
   *    FF03PPPP
   *
   * where PPPP is a PPP protocol type if we read it on a big-endian
   * machine.  0x0000 does not appear to be a valid PPP protocol type
   * value, so at least one of those hex digits is guaranteed not to
   * be 0.
   *
   * Old versions of libpcap for Linux used DLT_NULL for loopback devices,
   * but not any other devices.  (Current versions use DLT_EN10MB for it.)
   * The Linux loopback driver puts an *Ethernet* header at the beginning
   * of loopback packets, with fake source and destination addresses and
   * the appropriate Ethernet type value; however, those older versions of
   * libpcap for Linux compensated for this by skipping the source and
   * destination MAC addresses, replacing them with 2 bytes of 0.
   * This means that if we're reading the capture on a little-endian
   * machine, the header, treated as a 32-bit integer, looks like
   *
   *    EEEE0000
   *
   * where EEEE is a byte-swapped Ethernet type, and if we're reading it
   * on a big-endian machine, it looks like
   *
   *    0000EEEE
   *
   * where EEEE is an Ethernet type.
   *
   * If the first 2 bytes of the header are FF 03:
   *
   *    it can't be a big-endian BSD DLT_NULL header, or a DLT_LOOP
   *    header, as AF_ values are small so the first 2 bytes of the
   *    header would be 0;
   *
   *    it can't be a little-endian BSD DLT_NULL header, as the
   *    resulting AF_ value would be >= 0x03FF, which is too big
   *    for an AF_ value;
   *
   *    it can't be an IRIX or UNICOS/mp DLT_NULL header, as the
   *    resulting AF_ value with be 0x03FF.
   *
   * So the first thing we do is check the first two bytes of the
   * header; if it's FF 03, we treat the packet as a PPP frame.
   *
   * Otherwise, if the upper 16 bits are non-zero, either:
   *
   *    it's a BSD DLT_NULL header whose AF_ value is not in our
   *    byte order;
   *
   *    it's an IRIX or UNICOS/mp DLT_NULL header being read on
   *    a big-endian machine;
   *
   *    it's a Linux DLT_NULL header being read on a little-endian
   *    machine.
   *
   * In all those cases except for the IRIX or UNICOS/mp DLT_NULL header,
   * we should byte-swap it (if it's a Linux DLT_NULL header, that'll
   * put the Ethernet type in the right byte order).  In the case
   * of the IRIX or UNICOS/mp DLT_NULL header, we should just get
   * the upper 16 bits as an AF_ value.
   *
   * If it's a BSD DLT_NULL header whose AF_ value is not in our byte
   * order, then the upper 2 hex digits would be non-zero and the next
   * 2 hex digits down would be zero, as AF_ values fit in 8 bits, and
   * the upper 2 hex digits are the *lower* 8 bits of the value.
   *
   * If it's an IRIX or UNICOS/mp DLT_NULL header, the upper 2 hex digits
   * would be zero and the next 2 hex digits down would be non-zero, as
   * the upper 16 bits are a big-endian AF_ value.  Furthermore, the
   * next 2 hex digits down are likely to be < 0x60, as 0x60 is 96,
   * and, so far, we're far from requiring AF_ values that high.
   *
   * If it's a Linux DLT_NULL header, the third hex digit from the top
   * will be >= 6, as Ethernet types are >= 1536, or 0x0600, and
   * it's byte-swapped, so the second 2 hex digits from the top are
   * >= 0x60.
   *
   * So, if the upper 16 bits are non-zero:
   *
   *    if the upper 2 hex digits are 0 and the next 2 hex digits are
   *    in the range 0x00-0x5F, we treat it as a big-endian IRIX or
   *    UNICOS/mp DLT_NULL header;
   *
   *    otherwise, we byte-swap it and do the next stage.
   *
   * If the upper 16 bits are zero, either:
   *
   *    it's a BSD DLT_NULL header whose AF_ value is in our byte
   *    order;
   *
   *    it's an IRIX or UNICOS/mp DLT_NULL header being read on
   *    a little-endian machine;
   *
   *    it's a Linux DLT_NULL header being read on a big-endian
   *    machine.
   *
   * In all of those cases except for the IRIX or UNICOS/mp DLT_NULL header,
   * we should *not* byte-swap it.  In the case of the IRIX or UNICOS/mp
   * DLT_NULL header, we should extract the AF_ value and byte-swap it.
   *
   * If it's a BSD DLT_NULL header whose AF_ value is in our byte order,
   * the upper 6 hex digits would all be zero.
   *
   * If it's an IRIX or UNICOS/mp DLT_NULL header, the upper 4 hex
   * digits would be zero and the next 2 hex digits would not be zero.
   * Furthermore, the third hex digit from the bottom would be <
   */
  if (!BYTES_ARE_IN_FRAME(0, len, 2))
    return FALSE;

  if (pd[0] == 0xFF && pd[1] == 0x03) {
    /*
     * Hand it to PPP.
     */
    return call_capture_dissector(ppp_hdlc_cap_handle, pd, 0, len, cpinfo, pseudo_header);
  } else {
    /*
     * Treat it as a normal DLT_NULL header.
     */
    if (!BYTES_ARE_IN_FRAME(0, len, (int)sizeof(null_header)))
      return FALSE;

    memcpy((char *)&null_header, (const char *)&pd[0], sizeof(null_header));

    if ((null_header & 0xFFFF0000) != 0) {
      /*
       * It is possible that the AF_ type was only a 16 bit value.
       * IRIX and UNICOS/mp loopback snoop use a 4 byte header with
       * AF_ type in the first 2 bytes!
       * BSD AF_ types will always have the upper 8 bits as 0.
       */
      if ((null_header & 0xFF000000) == 0 &&
          (null_header & 0x00FF0000) < 0x00060000) {
        /*
         * Looks like a IRIX or UNICOS/mp loopback header, in the
         * correct byte order.  Set the null header value to the
         * AF_ type, which is in the upper 16 bits of "null_header".
         */
        null_header >>= 16;
      } else {
        /* Byte-swap it. */
        null_header = GUINT32_SWAP_LE_BE(null_header);
      }
    } else {
      /*
       * Check for an IRIX or UNICOS/mp snoop header.
       */
      if ((null_header & 0x000000FF) == 0 &&
          (null_header & 0x0000FF00) < 0x00000600) {
        /*
         * Looks like a IRIX or UNICOS/mp loopback header, in the
         * wrong byte order.  Set the null header value to the AF_
         * type; that's in the lower 16 bits of "null_header", but
         * is byte-swapped.
         */
        null_header = GUINT16_SWAP_LE_BE(null_header & 0xFFFF);
      }
    }

    /*
     * The null header value must be greater than the IEEE 802.3 maximum
     * frame length to be a valid Ethernet type; if it is, hand it
     * to "capture_ethertype()", otherwise treat it as a BSD AF_type (we
     * wire in the values of the BSD AF_ types, because the values
     * in the file will be BSD values, and the OS on which
     * we're building this might not have the same values or
     * might not have them defined at all; XXX - what if different
     * BSD derivatives have different values?).
     */
    if (null_header > IEEE_802_3_MAX_LEN)
      return try_capture_dissector("ethertype", null_header, pd, 4, len, cpinfo, pseudo_header);
    else
      return try_capture_dissector("null.bsd", null_header, pd, 4, len, cpinfo, pseudo_header);
  }

  return FALSE;
}

static int
dissect_null(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_)
{
  guint32       null_header;
  proto_tree    *fh_tree;
  proto_item    *ti;
  tvbuff_t      *next_tvb;

  /*
   * See comment in "capture_null()" for an explanation of what we're
   * doing.
   */
  if (tvb_get_ntohs(tvb, 0) == 0xFF03) {
    /*
     * Hand it to PPP.
     */
    call_dissector(ppp_hdlc_handle, tvb, pinfo, tree);
  } else {

    /* load the top pane info. This should be overwritten by
       the next protocol in the stack */
    col_set_str(pinfo->cinfo, COL_RES_DL_SRC, "N/A");
    col_set_str(pinfo->cinfo, COL_RES_DL_DST, "N/A");
    col_set_str(pinfo->cinfo, COL_PROTOCOL, "N/A");
    col_set_str(pinfo->cinfo, COL_INFO, "Null/Loopback");

    /*
     * Treat it as a normal DLT_NULL header.  Fetch it in host
     * byte order.
     */
    null_header = tvb_get_h_guint32(tvb, 0);

    if ((null_header & 0xFFFF0000) != 0) {
      /*
       * It is possible that the AF_ type was only a 16 bit value.
       * IRIX and UNICOS/mp loopback snoop use a 4 byte header with
       * AF_ type in the first 2 bytes!
       * BSD AF_ types will always have the upper 8 bits as 0.
       */
      if ((null_header & 0xFF000000) == 0 &&
          (null_header & 0x00FF0000) < 0x00060000) {
        /*
         * Looks like a IRIX or UNICOS/mp loopback header, in the
         * correct byte order.  Set the null header value to the
         * AF_ type, which is in the upper 16 bits of "null_header".
         */
        null_header >>= 16;
      } else {
        /* Byte-swap it. */
        null_header = GUINT32_SWAP_LE_BE(null_header);
      }
    } else {
      /*
       * Check for an IRIX or UNICOS/mp snoop header.
       */
      if ((null_header & 0x000000FF) == 0 &&
          (null_header & 0x0000FF00) < 0x00000600) {
        /*
         * Looks like a IRIX or UNICOS/mp loopback header, in the
         * wrong byte order.  Set the null header value to the AF_
         * type; that's in the lower 16 bits of "null_header", but
         * is byte-swapped.
         */
        null_header = GUINT16_SWAP_LE_BE(null_header & 0xFFFF);
      }
    }

    /*
     * The null header value must be greater than the IEEE 802.3 maximum
     * frame length to be a valid Ethernet type; if it is, dissect it
     * as one, otherwise treat it as a BSD AF_type (we wire in the values
     * of the BSD AF_ types, because the values in the file will be BSD
     * values, and the OS on which we're building this might not have the
     * same values or might not have them defined at all; XXX - what if
     * different BSD derivatives have different values?).
     */
    if (null_header > IEEE_802_3_MAX_LEN) {
      if (tree) {
        ti = proto_tree_add_item(tree, proto_null, tvb, 0, 4, ENC_NA);
        fh_tree = proto_item_add_subtree(ti, ett_null);
        proto_tree_add_uint(fh_tree, hf_null_etype, tvb, 0, 4,
          (guint16) null_header);
      }

      next_tvb = tvb_new_subset_remaining(tvb, 4);
      if (!dissector_try_uint(ethertype_dissector_table,
            (guint16) null_header, next_tvb, pinfo, tree))
        call_data_dissector(next_tvb, pinfo, tree);
    } else {
      /* populate a tree in the second pane with the status of the link
         layer (ie none) */
      if (tree) {
        ti = proto_tree_add_item(tree, proto_null, tvb, 0, 4, ENC_NA);
        fh_tree = proto_item_add_subtree(ti, ett_null);
        proto_tree_add_uint(fh_tree, hf_null_family, tvb, 0, 4, null_header);
      }

      next_tvb = tvb_new_subset_remaining(tvb, 4);
      if (!dissector_try_uint(null_dissector_table, null_header,
            next_tvb, pinfo, tree)) {
        /* No sub-dissector found.  Label rest of packet as "Data" */
        call_data_dissector(next_tvb, pinfo, tree);
      }
    }
  }
  return tvb_captured_length(tvb);
}

/*
 * OpenBSD DLT_LOOP; like DLT_NULL, but with the first 4 byte *always*
 * being a *big-endian* type.
 */
static int
dissect_loop(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_)
{
  guint32       loop_family;
  proto_tree    *fh_tree;
  proto_item    *ti;
  tvbuff_t      *next_tvb;

  /* load the top pane info. This should be overwritten by
     the next protocol in the stack */
  col_set_str(pinfo->cinfo, COL_RES_DL_SRC, "N/A");
  col_set_str(pinfo->cinfo, COL_RES_DL_DST, "N/A");
  col_set_str(pinfo->cinfo, COL_PROTOCOL, "N/A");
  col_set_str(pinfo->cinfo, COL_INFO, "Null/Loopback");

  /* populate a tree in the second pane with the status of the link
     layer (ie none) */
  loop_family = tvb_get_ntohl(tvb, 0);
  if (tree) {
    ti = proto_tree_add_item(tree, proto_null, tvb, 0, 4, ENC_NA);
    fh_tree = proto_item_add_subtree(ti, ett_null);
    proto_tree_add_uint(fh_tree, hf_null_family, tvb, 0, 4, loop_family);
  }

  next_tvb = tvb_new_subset_remaining(tvb, 4);
  if (!dissector_try_uint(null_dissector_table, loop_family,
        next_tvb, pinfo, tree)) {
    /* No sub-dissector found.  Label rest of packet as "Data" */
    call_data_dissector(next_tvb, pinfo, tree);
  }
  return tvb_captured_length(tvb);
}

void
proto_register_null(void)
{
  static hf_register_info hf[] = {

    /* registered here but handled in ethertype.c */
    { &hf_null_etype,
      { "Type",         "null.type", FT_UINT16, BASE_HEX, VALS(etype_vals), 0x0,
        NULL, HFILL }},

    { &hf_null_family,
      { "Family",       "null.family", FT_UINT32, BASE_DEC, VALS(family_vals), 0x0,
        NULL, HFILL }}
  };
  static gint *ett[] = {
    &ett_null,
  };

  proto_null = proto_register_protocol("Null/Loopback", "Null", "null");
  proto_register_field_array(proto_null, hf, array_length(hf));
  proto_register_subtree_array(ett, array_length(ett));

  /* subdissector code */
  null_dissector_table = register_dissector_table("null.type",
                                                  "Null type", proto_null, FT_UINT32, BASE_DEC);

  register_capture_dissector_table("null.bsd", "Null/Loopback BSD AF");

  null_handle = register_dissector("null", dissect_null, proto_null);
  loop_handle = register_dissector("null.loop", dissect_loop, proto_null);
  null_cap_handle = register_capture_dissector("null", capture_null, proto_null);
}

void
proto_reg_handoff_null(void)
{
  /*
   * Get a handle for the PPP-in-HDLC-like-framing dissector and
   * the "I don't know what this is" dissector.
   */
  ppp_hdlc_handle = find_dissector_add_dependency("ppp_hdlc", proto_null);

  ethertype_dissector_table = find_dissector_table("ethertype");

  dissector_add_uint("wtap_encap", WTAP_ENCAP_NULL, null_handle);

  dissector_add_uint("wtap_encap", WTAP_ENCAP_LOOP, loop_handle);

  capture_dissector_add_uint("wtap_encap", WTAP_ENCAP_NULL, null_cap_handle);
  capture_dissector_add_uint("wtap_encap", WTAP_ENCAP_LOOP, null_cap_handle);

  ppp_hdlc_cap_handle = find_capture_dissector("ppp_hdlc");
}

/*
 * Editor modelines
 *
 * Local Variables:
 * c-basic-offset: 2
 * tab-width: 8
 * indent-tabs-mode: nil
 * End:
 *
 * ex: set shiftwidth=2 tabstop=8 expandtab:
 * :indentSize=2:tabSize=8:noTabs=true:
 */